1. Field
The following description relates to a method for determining a transmission power in a multi-input multi-output (MIMO) system based on a cooperative transmission.
2. Description of Related Art
A MIMO system includes a plurality of antennas in a transmitter or a receiver to achieve a high gain. A transmission power used by the transmitter included in the MIMO system decreases proportionally to an increase in a number of antennas of the transmitter. Such a relationship is contrary to the fact that a signal-to-noise ratio (SNR) increases based on the increase in the number of the antennas.
Therefore, when the number of antennas of the transmitter is considerably increased, a reduction gain of the transmission power that meets a quality-of-service (QoS) of the receiver may be maximized. The aforementioned system may be defined as a massive MIMO system.
For efficient use of limited frequency resources, all wireless communication standards related to a base station-mobile phone communication define that cells communicate by sharing the same frequency. However, due to the cells sharing the same frequency, an inter-cell interference (ICI) may be caused, thereby limiting a capacity of the cells.
Accordingly, a cooperative MIMO system has been suggested, which supports a multi-user (MU) by cooperation among transmitters of respective adjacent cells. The transmitters are interconnected by a high-speed fiber-optic cable. Channel information of users of the respective adjacent cells and transmitted data are exchanged through the fiber-optic cable. Since the transmitters transmit data to a receiver by cooperating based on the exchanged information, the ICI may be effectively reduced, and the entire capacity of the system may be increased. Furthermore, the cooperation among the transmitters may be effective in view of a power gain, a channel gain, and the like. Therefore, the cooperative MIMO system has been used for coverage expansion by increasing a capacity of cell-edge users in the international mobile telecommunications-advanced (IMT-Advanced).
In the cooperative MIMO system, cooperative communication beamforming and power allocation of a transmitter are performed under an available power constraint. The available power constraint may be divided into two types, that is, a pooled-power constraint and a per-base power constraint. The pooled-power constraint enables transmitters participating in a cooperative transmission to freely share a total available power. The per-base power constraint limits a maximum power of each transmitter participating in the cooperative transmission.
However, when the power allocation is performed based on the pooled-power constraint, the transmitters may use an excessively high amount of power, thereby affecting adjacent cells with a high interference. Therefore, the per-base constraint has been used.
A beamforming scheme and a power allocation scheme that reduce a transmission power have been applied to a MU-MIMO system. However, an optimal beamforming scheme that minimizes a transmission power of a transmitter while meeting a QoS of each receiver in a multi cell environment, and a power allocation scheme for each beamforming, are needed.